School in Paris as a gentleman-cadet, with the rank of sous-lieutenant on 11 September 1781. When he graduated on 8 May 1784, he joined the Artois Regiment—later the 48th Infantry Regiment—as a lieutenant. He was present at Rennes during the turmoil immediately following the Revolution, particularly divisive in the military. Some of soldiers, and indeed of some of the officers, suspected the loyalty of fellow officers remained with the old regime. Hastrel worried that the officers could no longer command the obedience of the troops, and recounted in his journal: "The reception of the tri-color occasioned a scene of insubordination, which deprived us of several officers and served as a pretext to the revolutionaries to inspire defiance among the soldiers. At the moment when the flag was blessed and given to the battalion, the commander gave the orders to leave the church, but the soldiers would not budge. Stirred up by the youths who had assisted with the ceremony, they declared that they would not leave the old flag to be hung from the church vaults.... We could not enforce our commands....Seeing his authority ignored, the commander left the church and was followed by six or seven other officers. Then Capitaine Sermizelles, who had stayed, took the flag and gave it to the priest to be hung in the church.... [T]he battalion...returned to the barracks in order." He was promoted to captain with the campaign of the Vosges on 13 April 1792. He participated in the engagements at Frankfurt am Main and Hochheim and siege of Mainz (1793), and a small engagement at Bingen. Subsequently he was named adjutant staff major of the Army of the Rhine on 11 July 1793. In 1799, he was appointed to Jean-Baptiste Jourdan's general staff of the Army of the Danube, garrisoned initially in Strasbourg. After crossing the Rhine in early March, the Army engaged Archduke Charles' Austrian troops at the battles of Ostrach and Stockach. Following defeat at Ostrach, the army was reorganized with the Army of Helvetia, under command of André Masséna. Hastrel received a staff appointment in Milan. François Antoine Louis Bourcier served with Hastrel in the Army of the Danube. In 1800, he described Hastrel to François Nicolas Fririon: "The talents he has received from nature have been expanded by a careful education. His activities and services as the deputy of the general staff should make him eligible for promotion